A team of researchers has documented a recent volcanic eruption in the western Pacific Ocean about 2.8 miles (4.5 km) below the ocean surface that they describe as the deepest known eruption on Earth – deeper below the ocean surface than Mount Rainier’s height above sea level.

The researchers say the eruption probably happened between 2013-2015 on the Mariana back-arc, a zone of the sea floor with active volcanoes in the Pacific Ocean’s Mariana Trench. The Mariana Trench is the deepest part of the earth’s oceans, and the deepest location of the earth itself. It’s located just east of the 14 Mariana Islands near Japan. It was created by ocean-to-ocean subduction, a phenomenon in which a tectonic plate topped by oceanic crust is subducted beneath another plate also topped by oceanic crust.

Bill Chadwick is a marine geologist at Oregon State University and lead author on the study, published October 23, 2018 in the peer-reviewed journal Frontiers in Earth Science. Chadwick said in a statement:

We know that most of the world’s volcanic activity actually takes place in the ocean, but most of it goes undetected and unseen. That is because undersea quakes associated with volcanism are usually small, and most of the instrumentation is far away on land.

Many of these areas are deep and don’t leave any clues on the surface. That makes submarine eruptions very elusive.

The Mariana back-arc eruption was first discovered in December 2015 by cameras aboard an autonomous underwater vehicle. Photos revealed the presence of a pristine dark, glassy lava flow on the seafloor with no sediment cover. Venting of milky hydrothermal vent fluid indicated that the lava flow was still warm, and therefore very young.

Data indicated that there had been major depth changes in the area between surveys in 2013 and 2015, the researchers said, which is consistent with an eruption. The new lava flows stretched over an area about 4.5 miles (7.2 km) long and ranged in thickness between 130-450 feet (40-137 meters).

The scientists returned in April and December of 2016 and used two remotely operated vehicles to explore the site. The new observations showed a rapidly declining hydrothermal system on the lava flows, suggesting the eruption had taken place only months before its discovery the previous year. Chadwick said:

Typically after an eruption, there is heat released and venting for a few years and organisms will colonize the vents, creating a new ecosystem. But after a while, the system cools down and the mobile organisms will leave. There was still some venting, but it had obviously greatly declined.

Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as Lead Editor at EarthSky.org, she helps present the science and nature stories and photos you enjoy. She also serves as one of the voices of EarthSky on social media platforms including Facebook, Twitter and G+. She and her husband live in Tennessee and have two grown sons.